Preheat the oven to 220°C (200°C fan/425°F) and line a 12-cup muffin tin.
In a large mixing bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt. Then mix in the chocolate chips.
In a separate bowl or large measuring jug, combine the melted butter (slightly cooled), vegetable oil, sugars, eggs, vanilla, sour cream, and milk. Whisk until smooth and fully combined.
Make a well in the centre of the flour mixture. Pour in the wet ingredients and gently mix together until JUST combined, and you can't see any more streaks of flour. It's perfectly normal for the batter to be a little lumpy.
Divide the batter among the muffin cups, filling each about 0.3 cm (⅛ in) from the top. You should get 10-12 muffins depending on the size of your muffin tin.
Combine the course sugar with the cinnamon (if using). Sprinkle the sugar over the muffins, then top with extra chocolate chips.
Bake for 5 minutes. Then, without opening the oven door, reduce the temperature to 180°C (160°C fan/350°F), and bake for a further 15-18 minutes. The tops should be golden brown, and a toothpick inserted into the centre should come out clean (besides some melted chocolate).
Allow the muffins to cool in the tin for 5 minutes before transferring to a wire rack. Enjoy the muffins while still warm, or allow to cool completely before storing (see notes for storage instructions).